N-oxides hold a key position in the chemistry of heterocycles as well as in biomedical area. Tertiary amine oxides are widely used in treatment of fabrics and preparation of hair conditioners and shampoos, toothpaste, laundry detergent powder, fabric softeners, toilet soap bars and cosmetics as well as in other applications. They were also used as stoichiometric oxidants in metal catalysed hydroxylation and epoxidation reactions of olefins. On the other hand, oxides derived from secondary amines, called nitrones are highly valuable synthetic intermediates and excellent spin trapping reagents. In particular nitrones are excellent 1,3 dipoles and have been utilized for the synthesis of various nitrogen containing biologically active compounds e.g. alkaloids and lactams.
Conventionally tertiary amine oxides are prepared by oxidation of respective tertiary amines with strong oxidising agent like aqueous hydrogen peroxide in a solvent such as water, lower alcohol, acetone or acetic acid. A dilute or preferably concentrated (30-90% by weight) hydrogen peroxide solution is added in stoichiometric or greater amount to an aqueous solution containing the tertiary amine to obtain amine oxide, (U.S. Pat. No. 3,215,741). The drawback associated with this process is the formation of a gel resembling a thick paste long before completion of the reaction, which retards further reaction. The yields are only 30-40% by weight of amine oxide. Several other methods such as incorporation of catalyst and/chelating agent have been developed in order to increase the quality and yields of the product.
In case of secondary amines, the classical methods involve the condensation of N-monosubstituted hydroxylamines with carbonyl compounds or the direct oxidation of N,N-disubstituted hydroxylamines. Subsequently, direct oxidation of secondary amines using several oxidising systems such as R.sub.2 C(.mu.-O.sub.2), Na.sub.2 WO.sub.4 --H.sub.2 O.sub.2, SeO.sub.2, TPAP-NMO and UHP-M (M.dbd.Mo, W), MTO-H.sub.2 O.sub.2 have been developed to prepare nitrones under homogenous conditions. The drawback in all the above cases is the difficulty in recovering the homogeneous catalyst/reagents from the reaction mixture.
Reference is made to U.S. Pat. No. 3,283,007 wherein the oxidation of tertiary amines using diethelene triamine penta/tetra acetic acid as chelating agent and sometimes contaminated with heavy metals is recommended to improve the yield. The hydrogen peroxide solution employed has concentration of at least 30-75% by weight. The disadvantages of this process are high reaction temperatures ranging between 40-100.degree. C., longer reaction periods, and lower yields of amine oxides.
Reference is made to U.S. Pat. No. 3,424,780, wherein high yields of tertiary amine oxides are achieved by carrying the oxidation of tertiary amine with 30-70% by weight of aqueous hydrogen peroxide using 0.01 to 2% weight of carbondioxide, in presence of a chelating agent, tetra acetylene diamine, a salt thereof, polyphosphates, stannates, a hydroxy carboxylic acid salts or the salt of poly carboxylic acid. The reaction is carried out at a temperature ranging from 40 to 80.degree. C. The disadvantages of this process are high reaction temperature, longer reaction periods and that the amine oxide formed is intensively coloured when carbon dioxide atmosphere is used to speed up the reaction and this method necessitates injecting a gas which requires handling facilities. Another disadvantage is that the presence of more than 30% by weight of hydrogen peroxide is not environmentally friendly.
Reference is made to another U.S. Pat. No. 4,889,954 wherein the tertiary amines are reacted in high yields to give the corresponding amine oxides with a low content of nitrosamine, the oxidation of tertiary amine being carried out in the presence of a dialkyl carboxylic acid ester as catalyst and if appropriate, ascorbic acid as a co-catalyst using 45-70% by weight of hydrogen peroxide. The drawbacks in the above process are the requirement of frequent addition of water to avoid gel formation, high reaction temperatures, longer reaction periods and difficulty in separation of the catalyst from the reaction mixture.
Reference is made to another U.S. Pat. No. 4,565,891 wherein octacyano molybdate or iron salts are used as catalysts and molecular oxygen for oxidation of tertiary amines at high pressures and temperatures. The main drawback of this process is the need of very high temperature of 90-130.degree. C. and very low yields of amine oxide reporting 11-52% of conversion.
Reference is made to a U.S. Pat. No. 5,130,488 wherein the solid amine oxide can be prepared by reacting a tertiary amine with hydrogen peroxide using carbon dioxide in presence of acetate and cooling to precipitate the product. This process is superior to previously known methods of preparing amine oxides. However, its use can sometimes lead to cleavage of the solvents, plating on the walls of the vessel used for the precipitation, contamination of the product with residual peroxide, and or discoloration of the product.
Reference is made to a publication by Walter W. Zajac et al., J. Org. Chem.; 53, 5856, 1988 wherein the oxidation of secondary and tertiary amines using 2-sulfonyloxyxaziridines (Davis Reagents) were reported. The drawback of the above process is that the reagent is used in stoichiometric amounts.
Reference is made to a publication by Shun-Ichi Murahashi et al., J. Org. Chem.; 55, 1736, 1990 wherein sodium tungstate is used as catalyst for the oxidation of secondary amines. The drawback is the difficulty in recovery of the catalyst from homogeneous conditions.
Reference is also made to publication by Murraay et al., J. Org. Chem., 61, 8099, 1996 wherein methyltrioxorhenium was used as a catalyst in oxidation of secondary amines. The drawback is the difficulty in recovery of the catalyst.